A trimmer capacitors basically is a variable capacitor whose capacitance may be intentionally and repeatedly changed mechanically or electronically. Trimmer are often used in L/C circuits to set the resonance frequency, e.g. to tune a radio, or as a variable reactance, e.g. for impedance matching in antenna tuners. This is normally in pf values.

EVE-eVision Electronics has large range of trimmers capacitors also known variable capacitors.